In the telecommunications industry a recent development known as the Synchronous Optical NETwork (SONET) is currently being standardized by the Exchange Carriers Standards Association (ECSA) and the International Telegraph and Telephone Consultative Committee (CCITT). SONET is a new optical interface specification with enough flexibility to transport many different types of payloads (i.e. voice, data, video, and new services). The currently proposed SONET standard contains specific requirements for mapping signals from the existing digital hierarchy to the new SONET standard. The existing digital transmission hierarchy includes the following signals carriers and corresponding data rates: DS-1 at 1.544 Mb/s; DS-2 at 6.312 Mb/s; and DS-3 at 44.736 Mb/s [American National Standard for Telecommunications, "Digital hierarchy electrical interfaces", ANSI Doc. T1.102, August 1987]. The DS-1 signal is the basic building block of the telecommunication transmission network. Twenty-four voice channels can be encoded into a each DS-1 signal stream. The bit steams of smaller signal carriers can be fed into larger carriers. For example, four DS-1 signal carriers can be fed into one DS-2 carrier and 7 DS-2 Carriers can be fed into one DS-3 carrier.
DS-1 signals can be mapped into a standard SONET signal known as VT1.5 (1.728 Mb/s) through an established technique known as bit stuffing. Bit stuffing allows the slower rate DS-1 signal to be accurately mapped into the higher rate SONET VT 1.5 signal. Efficient mapping of DS-1 signals into SONET is critical because DS-1 transmission facilities represent a significant portion of the existing telecommunication transmission network. Circuits that provide the interface function between the existing multiplex hierarchy and SONET are currently being developed by telecommunications equipment manufacturers. One of the interfaces that will be introduced in the first generation of SONET based equipment is a DS-3 to 28 VT1.5 SONET circuit. In this circuit, a DS-3 signal is demultiplexed into 28 DS-1 signals in two steps, DS-3 to DS-2 and DS-2 to DS-1, and each DS-1 subsequently mapped into a VT1.5 signal as specified by the SONET standard [American National Standard for Telecommunications, "Digital hierarchy optical interface rates and formats specification", ANSI Doc. T1.105, Sept. 1988]. The DS-3 to 28 VT1.5 SONET circuit would be needed in all SONET-compatible multiplex and digital cross-connect systems equipment requiring VT 1.5 (i.e.DS-1) visability.
At each demultiplexing step, prior art systems have required Desynchronizer Phase-Lock Loops (PLLs) to reduce jitter, and thus control signal degradation. These PLLs increase the complexity of the DS-3 to 28 VT1.5 SONET interface circuit and thus increase the expense of manufacturing the circuit. The conventional understanding in the art has been that removal of the Desynchronizer PLLs increases the jitter resulting from the demultiplexing beyond the network jitter requirements.
The invention provides a method of demultiplexing and an apparatus for demultiplexing the digital signal carriers without desynchronizer phase-lock loops while meeting jitter standards, and in one aspect comprises a process for the mapping of a first digital signal carrier carrying a first digital signal to a second digital signal carrier comprising; demultiplexing the first digital signal to produce a second digital signal; deriving a gapped clock from the first digital signal carrier; and writing the second digital signal to an elastic store using the gapped clock.
Further summary of the invention is found in the claims.